In addition to conventional speech transmission, modern mobile communication systems offer various data transmission services to subscribers. The services of mobile communication systems are generally classified into tele-services and bearer services. A bearer service is a telecommunications service which provides signal transmission between the user interfaces and the network interfaces. An example of bearer services is a modem service. In the tele-service terminal services are also offered by the network. Important tele-services include speech, telefax and videotex services. The bearer services are usually subdivided into groups, such as asynchronous bearer services and synchronous bearer services, on the basis of a certain feature. In the case of the asynchronous bearer service the transmitting terminal and the receiving terminal are able to maintain their synchronization only for each single character to be transmitted. In the case of the synchronous bearer service the transmitting terminal and the receiving terminal are synchronized with each other for the whole duration of data transmission. Each such bearer service group comprises a number of bearer services, such as a transparent service and a non-transparent service. In the transparent service the data to be transmitted is unstructured and transmission errors are corrected only by means of channel coding. In the non-transparent service the data to be transmitted is structured into protocol data units (PDU) and transmission errors are corrected using retransmission protocols (in addition to channel coding).
Calls have traditionally been calls with one service (one connection), i.e. each call has been clearly e.g. a speech call or a data call of a certain type and optimized for a certain service. Multimedia calls which simultaneously support various types of information transmission or services, such as video, speech, file transfer, have recently been introduced in fixed data networks, particularly in the Internet.
Existing mobile communication systems do not offer any special bearer services for multimedia calls or for simultaneous use of multiple data services. There is only one traffic channel available for a data call, the channel being either transparent (T) or non-transparent (NT). Depending on the required transmission rate a traffic channel may consist of one sub-channel (e.g. TDMA time slot) or several sub-channels (e.g. several TDMA time slots for high-speed data transmission, such as HSCSD in the GSM system). Any shared use of the traffic channel has to be implemented on the application layer, i.e. in the end user's applications. Time-critical multimedia calls, e.g. video phoning, have to use transparent circuit-switched bearer services because other data services cannot guarantee as small variation of the transmission delay as the video service requires. Too long a transmission delay causes visible interference in the video picture at the receiving end. Applications which are not time-critical and require accurate transmission usually use non-transparent bearer services. An example of such an application is transfer of data files.
As was stated above, the problem related to the present mobile communication systems is that they either provide a transparent or non-transparent traffic channel or a packet service (such as GPRS, General Packet Radio Service) for a multimedia call between two mobile stations or between a mobile station and a terminal or server in a fixed communication network. Packet radio services and the non-transparent traffic channel are not suitable for video phoning or other time-critical applications. On the application layer the transparent bearer service requires an error correction protocol which is usually not optimized for a radio connection. This means that a multiservice and/or multimedia channel has to always use a transparent bearer service and perform multiplexing and error correction on the application layer in the end users' terminals.
In the future it will be required of the mobile communication systems, particularly of the 3rd generation systems, such as UMTS (Universal Mobile Telecommunications Systems) that the mobile communication network and mobile station should support several simultaneous calls between the mobile station and several other parties at different destinations. The calls may be traditional calls with one connection or above-described multimedia calls or calls with multiple connections. It is also required that it should be possible to add or drop different connections or calls independently of one another.
The key factor in the design and implementation of mobile communication networks is the effective utilization of the radio spectrum. Multiple calls should also be implemented by making the maximum use of the channel capacity. Control of multiple calls, e.g. handover, should be as simple as possible both for the mobile communication system and the mobile station.